Gastric disease is an important cause of morbidity and economic loss in swine-rearing operations (O'Brien, J. (1992) “Gastric ulcers” p. 680. In A. D. Leman, B. E. Straw, W. L. Mengeling, and S. D. D'Allaire (ed), Diseases of swine. Wolfe, London, United Kingdom). Although the cause of porcine gastric disease has not been previously established, it is most often attributed to diet and/or stress (O'Brien, J. (1992) “Gastric ulcers” p. 680. In A. D. Leman, B. E. Straw, W. L. Mengeling, and S. D. D'Allaire (ed), Diseases of swine. Wolfe, London, United Kingdom).
In 1984, Helicobacter pylori (Hp) emerged as an etiologic agent in human gastritis/ulcer disease following the documentation of this agent in patients with gastritis (Marshall and Warren (1984) Lancet 1:1311-1314). Hp is a Gram-negative microaerophilic urease-positive small curved rod-shaped bacterium which possesses several unusual characteristics related to its gastric ecological niche. The hallmark for the members of the Helicobacter genus is expression of urease enzyme. The presence of this enzyme and its activity in the hydrolysis of urea forms the basis of presumptive tests (urea breath test and others) for gastric colonization. The organism colonizes the mucus layer of the gastric cardia and antrum and infection is presumed to be lifelong.
Hp is now universally recognized as one of the primary gastric pathogens, and the study of this bacterial species and the spectrum of diseases associated with it has become a major focus in human gastroenterology (Suerbaum and Michetti (2002) N. Eng. J. Med. 347:1175-1186). Hp is causally associated with chronic superficial (active) type B gastritis (Buck (1990) Clin. Micro. Rev. 3:1-12; Blaser (1992) Gasteroenterol. 102:720-727; Consensus Statement, 1994, NSAID), independent gastric ulceration (Peterson (1991) N. Eng. J. Med. 324:1043-1047; Moss and Calam (1992) Gut 33:289-292; Leung et al. (1992) Am. J. Clin. Pathol. 98:569-574; Forbes et al. (1994) Lancet 343:258-260), atrophic gastritis (Nomura et al. (1991) N. Engl. J. Med. 325:1132-1136; Parsonnet et al. (1991) JNCI 83:640-643; Sipponen (1992) Drugs 52:799-804, 1996), and gastric MALT lymphoma (Rodriguez et al. (1993) Acta Gastro-Enterol. Belg. 56(suppl):47; Eidt et al. (1994) J. Clin. Pathol. 47:436-439). Additionally, atrophic gastritis and resultant acholrhydria is now thought to represent the last stage in the progression of persistent lifelong colonization by Hp (Leung et al. (1992) Am. J. Clin. Pathol. 98:569 574).
Early attempts to reproduce disease with Hp were frustrated because commonly used laboratory animal species were found to be highly resistant to Hp gastric colonization. Experimental animal models of Hp gastritis infection have since been developed. Prominent among these is the gnotobiotic piglet model for acute bacterial gastritis (Krakowka et al. (1987) Infect. Immun. 55:2789-2796). Gnotobiotic swine, as monogastric omnivores whose gastric anatomy and physiology most closely replicates humans (Bertram et al. (1991) Rev. Infect. Dis. 13:S714-S722), are susceptible to oral colonization with many different Hp strains (Krakowka et al. (1987) Infect. Immun. 55:2789-2796; Bertram et al. (1991) Rev. Infect. Dis. 13:S714-S722). Lewis antigenic arrays expressed on human mucoproteins and cell surface glycoproteins are thought to be binding receptors for Hp bacterial lipopolysaccharide and other surface glycoproteins (Vandenbroucke-Grauls et al. (1998) Ital. J. Gastroenterol. Hepatol. 30:259-260). Swine gastric tissues, unlike other laboratory animal species except primates are also Lewis antigen-positive (Appelmelk et al. (1998) “Molecular Mimicry between Helicobacter pylori and the host” in Helicobacter pylori: Basis Mechanisms to Clinical Cure (1998) R. H. Hunt, Tytgat G N T, eds.).
Of the domestic animal species, swine are the most commonly affected with clinically significant gastric ulcers (O'Brien J. J. Gastric Ulcers. Diseases of Swine, 6th ed. Editors A D Leman A D, et al., (1986), 680-691; Embaye et al. (1990) J. Comp. Path. 103:253-264). In modern swine-intensive production systems, the development of ulcers and erosions of the nonglandular esophageal (cardiac) gastric lining and antral gastric mucosa is a common and serious disease problem (O'Brien J. J. Gastric Ulcers. Diseases of Swine, 6th ed. Editors A D Leman A D, et al., (1986), 680-691). A prevalence of 5-100% for gastroesophageal ulcerations (GEU) is reported, death losses from fatal hemorrhages of 3% or more are reported (O'Brien J. J. Gastric Ulcers. Diseases of Swine, 6th ed. Editors A D Leman A D, et al., (1986), 680-691; Embaye et al. (1990) J. Comp. Path. 103:253-264) and sublethal economic losses are substantial.
Porcine gastric mucosal ulceration and GEU are attributed to reflux of acidic gastric contents onto the unprotected pars esophagea (Argenzio et al. (1975) Am. J. Physiol. 228:454-462; Argenzio et al. (1996) Am. J. Vet. Res. 57:564-573). In particular, the stratified squamous epithelium of the pars esophagea is devoid of mucous-producing glands and lacks the sodium bicarbonate buffering system characteristic of the gastric glandular mucosa and, as a consequence, the pars is frequently damaged by the acidic contents of the stomach. Elevated gastric acid content is multifactorial and thought to be largely due to a combination of excess parietal cell production of hydrochloric acid, luminal hydrolysis of luminal carbohydrate, both coupled with a loss of pH gradient in the stomachs of swine fed a finely ground low roughage high carbohydrate diet.
Feeder swine diets contain unsaturated fatty acids, short chain (acetate, propionate, butyrate and lactate) free fatty acids or peroxidized fats, all of which elevate luminal acid concentration (Argenzio et al. (1975) Am. J. Physiol. 228:454-462). Finishing diets high in carbohydrate such as corn and cornstarch are also a primary dietary source of acidic metabolites in pigs. Incomplete glycolysis of cornstarch by parietal cell-origin hydrogen ions and/or enzymatic actions of commensal fermentative microbes such as the Lactobacillus and Bacillus spp. results in the generation of lactic, acetic and propionic acids within the gastric compartment. Indeed it has been demonstrated that gastric colonization with fermentative bacterial species resulted in GEU if a dietary source of carbohydrate (corn syrup) was provided to colonized gnotobiotes (Krakowka et al. (1998) Vet. Pathol. 35:274-282). Finally, in feeder swine, the physical form of diet also influences development of GEU (O'Brien J. J. Gastric Ulcers. Diseases of swine, 6th ed. Editors A D Leman A D, et al., (1986), 680-691). In general, a finely ground (<3.5 mesh) diet, even in pelleted form is an important risk factor for ulcerogenesis presumably because of the general inability of these diets to “confine” released acids to the fermentation compartment of the glandular stomach. The loss of a pH gradient associated with finely ground diets permits cranial acid reflux into the pars esophagea.
As in humans with recurrent “heart burn,” reflux esophagitis and Barrett's esophagus, it is believed that gastric-origin hydrogen ions and acidic metabolites of partial intragastric glycolysis enter and acidify the squamous epithelial cell cytoplasm. The cell membrane-bound Na—K-ATPase is disrupted which results in accumulation of intracellular sodium ions and secondary accumulation of intracellular water, recognized histologically as acute cellular swelling, hydropic degeneration, epithelial parakeratosis and ultimately necrosis. For erosive lesions, the underlying basement membrane remains intact and re-epithelization of the damaged portion of the pars is rapid. Presumably pivotal to progression of epithelial erosions to ulceration is penetration of the basement membrane and continued acid-mediated damage to the underlying lamina propria. This devitalized tissue may be secondarily colonized by commensal microbes including fermentative anaerobes. In humans as well as swine, there is a strong consensus that a relative or absolute increase in gastric hydrogen ions (acid) is the proximate cause of pars and esophageal damage. Thus, a therapeutic goal in humans is to elevate gastric pH towards neutrality through the use of bicarbonate buffering medications and to inhibit new gastric hydrogen ion production by parietal cells of the gastric fundus with proton pump inhibitors. These over-the-counter medications provide immediate symptomatic relief for patients affected with heart burn and reflux esophagitis and indirectly implicate gastric hyperacidity in the pathogenesis of disease. However, such medications do not cure the underlying cause of the disease.
Attempts to treat Hp infection in humans using immunotherapy rather than chemotherapy has been largely unsuccessful. In particular, induction of immunity which mimics the “natural” immune response of convalescent infected humans has not been successful, since human Hp infection can persist indefinitely in spite of a strong immune response to Hp (Lee (1996) Gastroenterol. 110:2003-2006). In mice, protection has been achieved with sonicates or recombinant proteins such as ureA and ureB, vacA and GroEL, given orally with cholera toxin (CT) and heat labile toxin (LT) as adjuvants. The focus has been primarily upon the use of purified and/or recombinant bacterial proteins as target immunogens in vaccine development programs. In general, inconsistent and only partial protection has been achieved. In rodent systems, mucosal vaccination assisted by CT or LT has emerged as the favored route, notwithstanding the fact that these species are highly resistant to toxic effects of CT/LT and the resultant rodent data does not directly translate into the human or swine experience.
In particular, in piglets immunized and then challenged with Hp, the strongest pre-challenge indicator of efficacy is the level and presence of Hp-specific serum/salivary IgG, not IgA (Eaton and Krakowka (1992) Gastroenterol. 103:1580-1586). Parenteral vaccination stimulates a strong IgG response; oral vaccination does not. Parenteral immunization was completely protective in 50% of the piglets immunized subcutaneously and in 60% of piglets immunized intraperitoneally (Eaton et al. (1998) J. Infect. Dis. 178:1399-1405). In contrast, oral vaccination with: 1) live bacteria (cleared with antimicrobials prior to challenge), 2) whole intact killed bacteria, 3) whole bacterial sonicates and 4) whole bacterial sonicates with mucosal LT adjuvant failed to provide a single instance (0 of 27 piglets or 0%) of protection. Bacterial cfu were reduced compared to controls but the levels of reduction did not reach statistical significance. Thus, in the porcine model of Hp colonization and acute gastritis, the parenteral route of vaccination appears to be superior to the oral route in both absolute (infected versus uninfected after challenge) and relative (bacterial cfu in vaccinates versus nonvaccinated controls) measures of antimicrobial efficacy.
Multiple agent antimicrobial therapies have been available for human Hp for more than a decade. These therapies can be expensive, cumbersome to administer, and often do not completely cure the disease. Such therapies would be impractical in domestic livestock. Moreover, injudicious use of antimicrobials promotes emergence of antibiotic-resistant strains of Hp and Hp resistance to metronidazole and clarirythromycin has increased (Michetti, (1997) Gut 41:728-730). Additionally, the use of antibiotics in food animals is undesirable. Thus, there is a continuing need for discovering new modes of preventing or treating Helicobacter infection in both humans and animals. Animal models that mimic Helicobacter infection are of great use in studying treatment and prevention options.
Recently, a new Helicobacter pathogen was recovered from swine exhibiting gastritis/ulcer disease. This pathogen, named H. cerdo, has been shown to cause gastric disease in young piglets that is similar to Hp-associated active gastritis in humans.